All types of optical fibers become lossy when bent. As the radius of bend curvature is decreased more and more light is lost. As the length of fiber bent into a curved shape increases, there is a corresponding increase in optical power loss that increases with the length of bent fiber and the reciprocal of the bend radius. These factors limit the radii of bends allowable in an assembly of fiber optic components.
Bending or twisting an optical fiber creates stresses in the fiber which lead to increased susceptibility to stress-induced failure. As the bend radius is decreased or the degree of twisting is increased, these stresses may lead to abrupt breakage. Fibers which are bent in relatively large radii are also subject to asymmetrical internal stresses and may fail after some unpredictable time period.
These forms of fiber breakage are of considerable concern to component and system designers. The size and physical layout of stationary fiber optic components and system elements is dictated to a large degree by the space needed to route fibers around bends without incurring excessive optical power loss or enhancing the chance of long-term stress-induced breakage.
In some applications it is necessary to provide flexible mechanical couplings between two or more devices interconnected by optical fibers. In such cases the interconnecting fiber is bent or twisted dynamically. When a fiber is bent dynamically the intensity of the optical signal passing through the bend is modulated. Compensating for the time varying bending loss modulation of signal power complicates the design of systems and may, in fact, prove impossible. For example, fiber optic sensors mounted on moving machine parts such as the arm of an industrial robot must be connected to optical sources and receivers by an optical fiber. Motions of the robot arm cause bends and bend related optical power loss in the interconnecting fiber. Hence the received signal from the sensor varies as the arm moves. If these variations exceed the dynamic range of the sensor instrumentation, compensation is not possible.
In other applications it is advantageous to use fibers to interconnect optical elements which may rotate relative to one another. Optical fibers in these cases are subjected to twisting. Considerable effort has been expended to develop rotary fiber optic joints to meet such demands. Prior efforts have resulted in an expensive component which requires extreme precision of manufacturing to ensure fiber alignment. Typically a rotary joint consists of two main parts that are free to rotate relatively about a common longitudinal axis. In each of these parts, an optical fiber is rigidly mounted precisely on the center of the axis of rotation. When assembled these two parts act to butt the ends of the fibers together and align the cores of the two separate fibers. When the two parts are rotated, the cores of the butted fibers must remain in alignment or variations of the signal power result, again, such variations in signal power are troublesome.
A particularly important application of optical fiber is in the fabrication of telecommunications circuits, networks and devices. An example is a fiber optic transceiver used to send and receive data on optical fibers. Typically such a transceiver comprises a set of electronic circuits which perform the functions of coupling an external device such as a computer workstation or telephone modem into a power amplifier suitable to drive a laser diode or LED light source. Other circuits control the bias point of the light source and may control the temperature of the light emitting junction. These elements of electronics comprise the transmitter portion of the transceiver. The result is that light is emitted according to desired power, optical wavelength, data bandwidth, and optical bandwidth criteria. This optical signal is then coupled into an optical fiber and routed in some manner out of the package enclosing the circuitry.
Other circuits in a typical transceiver package include a photodetector and amplifier to recover the optical signal fed into the package on an optical fiber. Once amplified these signals are usually conditioned by other circuits to match the required electrical input/output characteristics of the user's terminal equipment.
In many situations the same fiber is used to carry the optical signal in and out of the transceiver. A means of optical beam splitting is employed to separate the outgoing optical signal from the incoming optical signal. One such beam splitting means of current popularity is the fiber optic coupler. A fiber optic coupler can be made to be as small as about 2 cm long by roughly 5 mm in diameter. The electronics for the transceiver can be fabricated using standard dual in-line package (DIP) technology with the result that the electronics occupy about 2 cubic inches. It is difficult to configure a package using an internal fiber optic coupler because the radius of bends in the fibers entering and leaving the coupler must be greater than 1 cm. Careful circuit board layout can, however, result in a package of shape and volume dictated by the electronic component. Such a package has considerable application, but it is limited. For example, it would not fit neatly into any modern telephone handset.
For size reasons alone, many electronic functions are fabricated using miniaturized devices. Surface mount device (SMD) technology is becoming very popular for the advantages it offers in size reduction. With the size reduction also comes an increased speed ability. SMD technology and the similar hybrid circuit technology are well suited to data rates in excess of 50 megabits per second. Fiber optics are also suited to high data rate applications. It is quite natural that designers seek to incorporate the advantages of miniaturized circuit technologies with optical fiber systems. The electronics for a transceiver like the one described above, when built using SMD occupies less than 0.75 cubic inches. There is, however, no previously known way to route the fibers inside the package without bends that increase the volume unacceptably, therefore, the fiber optic coupler becomes the limiting item in final package size.
It is therefore desirable to provide a means by which optical fibers can be bent or twisted without incurring excessive losses or susceptibility to stress related breakage.